50 THF. ROYAL SOCIETY OP^ CANADA 



T. canadensis. Except for the tubercle they could as well be ascribed 

 to T. glaber (Figure 4). 



The great resemblance of T. canadensis to T. glaber has already 

 been referred to. In order to carry the comparison further three 

 of Billings' cotypes of T. glaber were obtained from Ottawa through 

 the kindness of Dr. E. M. Kindle. One of these cotypes w^hich 

 Billings evidently used for the head of T. glaber is very imperfectly 

 preserved with the free cheeks pushed inwards on both sides and 

 forward on the right. There is no indication of a genal spine, but, in 

 view of the state of preservation, it cannot confidently be stated that 

 there was no spine. This specimen, No. 1939h, Geol. Sur. Canada, is 

 shown in Figure 5. 



The two other cotypes, Nos. 1939 and 1939e, Geol. Sur. Can., 

 were evidently used for the description of the body and show nothing 

 as to the character of the genal angle. In the collections of the Royal 

 Ontario Museum, however, is a specimen. No. 205 U, which is ab- 

 solutely identical with these two cotypes but which reveals the 

 character of the genal angle on one side. There is a distinct short 

 spine, outwardly directed as in 7\ canadensis; it is, however, less 

 developed and arises a short distance anterior to the genal angle 

 proper. Figure 6. 



The conclusion seems to be justified, therefore, that we may 

 expect to find eventually a whole series of specimens showing a gradual 

 transition from typical spineless T. glaber to the strongly spined 

 T. canadensis. It is probable, also, that this series will be an ascending 

 one geologically, as the Rouge exposures with typical 7\ canadensis 

 are undoubtedly very near the top of the Utica. 



Triarthrus spinosus was described by Billings in the Report of the 

 Geological Survey of Canada for 1853-56. The species is characterized 

 by a spine on the occipital segment of the head reaching to the third 

 or fourth thoracic ring, a spine on the eighth thoracic ring reaching 

 beyond the pygidium, and genal spines reaching to the seventh or 

 eighth pleurae. 



Ami drew attention to the many \-ariations in this species observed 

 by him on specimens from Cummings' bridge near Ottawa — the 

 occurrence of a spine on the ninth instead of on the eighth segment of 

 the thorax; spines on segments eight, nine, and ten; and a tubercle in 

 front of the origin of the spine on the neck segment.^ 



A great many free cheeks undoubtedly belonging to T. spinosus 

 occur in the exposures on the Rouge. With these are associated 



I Ottawa Naturalist, Vol. I, No. 3, p. 64, 1882. Ibid, No. 4, p. 88, PI. I, 1883 



